In general, there are three commonly used methods for manufacturing printed circuit boards. The methods are broadly termed subtractive, fully additive and semi-additive. The characteristics of each of these processing methods is amply discussed in U.S. Pat. No. 4,847,114 (Brasch et al.), the teachings of which are incorporated herein by reference in their entirety.
Improvements in these general processes are widespread. One avenue that improvements have concentrated is that of minaturization. Various modifications to the general processes indicated above have been introduced in order to enable the creation of more circuitry and interconnects per unit of surface area.
One such improvement is described in U.S. Pat. No. 5,246,817 (Shipley, Jr.), the teachings of which are incorporated herein by reference in their entirety. Shipley, Jr. discusses a process for the formation of multilayer circuit boards where layers are formed sequentially using selective plating techniques and imaging of dielectric materials to achieve fine line resolution and interconnections between circuits, thereby permitting multilayers of higher density using imaging techniques.
Thus the techniques discussed in Shipley, Jr. involve the deposition of electroless copper onto the surface of the imaged dielectric material. In fact Shipley, Jr. specifically discusses the use of electroless copper solutions to achieve metallization of the imaged dielectric at column 14, lines 10 to 23. The electroless copper solutions referred to in Shipley, Jr. are typical electroless copper solutions, preferably with the ability to plate ductile copper, capable of withstanding a solder shock test of about 260.degree. C. for 10 seconds without cracking.
Several problems have been experienced in practicing the methods discussed in Shipley, Jr. One such problem is in achieving substantial coverage of the imaged dielectric surface with the plated copper. A second difficulty occurs in achieving good functional adhesion of the plated metal to the imaged dielectric surface. These problems are particularly pronounced when the imaged dielectric surface is polyimide, polyetherimide or blends of either of the foregoing resins with each other or with other resins.